In the field of high-speed printing devices which are especially suitable for use in connection with electronic business systems, the wire matrix type of printer has come into increasing use. In this type of printer, letters, numbers and symbols are formed from a series of dots produced by the impact of the ends of a plurality of wire elements on record media.
Customarily, each of the individual wire printing elements of a wire matrix printer is driven by a solenoid which is energized when the printing stroke of that wire is required. To activate the solenoid quickly a high voltage, generally a square wave, is applied to the solenoid. This in turn increases the current through the solenoid at a rapid rate. As the current increases, the I.sup.2 R, or heating losses, also increase. The magnitude of current flowing through the solenoid for an entire print cycle is generally excessive in two ways, one; the current causes excess heating which in turn could cause destruction of the solenoid, and two; the power consumed would be greater than the power necessary to perform desired function.
A number of prior art techniques have been used in an attempt to minimize these particular problems. One such technique is disclosed in U.S. Pat. No. 3,237,088 entitled "Current Regulator For Inductive Loads", by J. R. Carp et al. The disclosed regulator circuit utilizes a resistive load placed in series with the inductor for sensing the current flowing through the inductor and for developing a voltage which is proportional to the sensed current. The voltage is fed back to a voltage comparator circuit which circuit compares the level of the developed voltage against a preselected level and provides an output indicative of the difference therebetween. The output from the voltage comparator is used to control a power amplifier which amplifier supplies the level of the voltage applied to the inductive load. The circuit disclosed in the reference patent recognizes that in order to obtain rapid changes in current through an inductor a large voltage must be available. When the current reaches the desired value, the voltage across the load inductor must be reduced to exactly the amount of IR drop in the load in order to sustain a constant load current. In addition, as previously stated, the amount of IR drop must not be allowed to cause excessive heating which in turn will cause premature failure of the inductor.
An additional prior art device of interest is disclosed in U.S. Pat. No. 3,549,955, entitled "Drive Circuit For Minimizing Power Consumption In Inductive Load", by T. O. Paine. The circuit of that patent connects the solenoid in series with a transistor switch and a resistor. The potential applied across the solenoid is controlled in its "on" and "off" state by the transistor switch. A voltage comparator monitors the voltage across the series resistor to compare the sensed voltage with a first threshold level voltage which first level is related to the pull-in current level of the solenoid. Once the solenoid has been activated the reference voltage is compared against a second threshold level, which second threshold is related to the drop-out current level of the solenoid. The transistor switch is alternately opened and closed to maintain the level of the current through the solenoid at a magnitude which is greater than the drop-out current, but substantially less than the initial pull-in current. This technique therefor minimizes the amount of power necessary to hold the solenoid in the activated position once initial pull-in is achieved.
An additional circuit of interest is disclosed in the co-pending application filed on even date herewith, U.S. Patent application Ser. No. 693,034, entitled "Solenoid Driver Circuit" by J. W. Stewart, the present inventor. The circuit of the co-pending application applies a differential voltage across the actuator solenoid and senses the current flowing through the solenoid to provide an indication of its level. When the current level reaches the desired maximum level the differential voltage is chopped off for a fixed interval of time and the current in the solenoid is allowed to decay at a controlled rate. By accurately controlling the off time of the differential voltage, increased regulation of the fluctuation of the current through the inductor is achieved.